Modern heating and cooling systems contain electrical control circuits that are interconnected and interlocked with the various sys- tem components by a series of switches and relays. Most of these components, particularly the heating and cooling equipment (furnaces, boilers, compressors, condensers, and so on) and most sys- tem controls have been described in other chapters of the book. This chapter is reserved for a description of the fan and limit controls; the various electrical control circuit switches and relays; transformers; and a number of different control devices used in cooling systems.
Fan Controls
A number of different devices are available for controlling the operation of fans in heating and/or cooling installations. Most of these devices function as fan safety controls; a few of them serve as fan primary controllers. The following are fan controls described in this chapter:
• Fan control
• Air switch
• Fan relays
• Fan center
• Fan manager
• Fan timer switch
• Fan safety cutoff switch
Caution
Always disconnect the power supply before installing, servicing, or repairing any of the electrically operated devices described in this chapter. Failure to do so may result in damage to equipment and/or electric shock.
Fan Control
A fan control is a device used to turn the system fan on and off in response to air temperature changes in the furnace plenum. This fan controller is frequently combined with a limit controller in one unit (see Combination Fan and Limit Control in this chapter).
In the operation of the furnace, the burner or burner assembly starts first and heats the air, which rises through the heat exchanger to the furnace plenum. The fan control is located in the plenum and is present for a specific cut-in temperature. When the temperature of the rising air reaches the cut-in temperature setting on the fan controller, the fan is automatically turned on and warm air is moved through the distribution ducts. After a period of time, the room thermostat will no longer call for heat and will shut off the burner. The air in the plenum then begins to cool. When the air temperature drops below the cut-in temperature of the fan controller, the fan is automatically shut off.
In most forced warm-air heating systems, the fan control is usually a line voltage device wired in the hot lead (L1) of the power supply to the fan motor (see Figure 6-1). If a step-down transformer is used to provide a low-voltage control circuit for the room thermostat, the fan motor and fan controller will connect at the line side of the transformer (see Figure 6-2).